In manufacturing, control, data processing and other fields, it is often desirable to generate signals representative of the force or pressure exerted on an object by a person, machine part or robot member. A response proportional to the pressure may then be obtained or the machine part or robot member can then be adjusted so that the object is handled properly. Such force sensing may also be used to determine the shape or position of an object by means of the pressure distribution over the sensor surface. In a robotics application, a force sensor is preferably located on the extremity of a gripping element. Such a sensor should be rugged, be adaptable to the shape of the element on which it is mounted and have spatial resolution compatible with its intended uses.
Several types of tactile type force sensors have been developed. The arrangement described in U.S. Pat. No. 4,286,459 issued to W. S. N. Trimmer et al on Sept. 1, 1981 utilizes a flexible piezoelectric material having conductive coatings. A force applied to the material causes a change in dimension so that the frequency of oscillation of the piezoelectric material is a function of the applied force. A system disclosed in U.S. Pat. No. 4,306,148 issued to C. G. Ringwall et al on Dec. 15, 1981 determines the air flow through an array of pneumatic flow passages by directing a beam of light on metallic tabs mounted on an elastic pad in the air stream and monitoring the light reflected therefrom. The force directed against selected portions of the elastic pad can thereby be detected. U.S. Pat. No. 3,979,711 issued to M. G. Maginness et al on Sept. 7, 1976 discloses an ultrasonic array and imaging system that may selectively scan an object to provide a detailed image over an extensive area.
A more direct approach to tactile sensing is described in the article "A Force Transducer Employing Conductive Silicone Rubber", pp. 73-80, by John A. Purbrick, Proceedings of the First International Conference on Robot Vision and Sensory Controls, published in 1982 by IFS Ltd., Bedford, England. The arrangement suggested therein utilizes a two-layer array each of which comprises a set of parallel silicone rubber bars. The bars of one layer are orthogonally oriented with respect to the bars of the other layer so that a grid of crosspoints is defined. The force exerted on each crosspoint determines the electrical resistance through the path including the crosspoint and the bar pair associated therewith. Other resistive tactile sensing devices use various configurations of conductors and deformable resistive sheets to sense the force exerted on portions of the device. It has been observed, however, that preferred materials such as silicone rubber exhibit non-linear and hysteresis characteristics that affect resistance measurements reflecting the pressure being sensed and that electrical connections to the resistive sheets adversely affect the accuracy of the measurements. The resulting distortions of force distribution have limited the utility of such resistive tactile sensors.
It is well known that the capacitance between a pair of conductors varies inversely as the distance between the conductors. This principle has been employed in the construction of strain gauges to measure applied forces and has even been extended to fingerprint sensing as disclosed in U.S. Pat. No. 4,353,056 issued to C. Tsikos on Oct. 5, 1982. The Tsikos arrangement includes a flexible insulator sandwiched between a flexible electrode and a flat sheet which comprises a large number of small spaced flat metal plates. Each plate is separately connected to multiplexing apparatus so that the voltage distribution across the plates can be measured. In this manner, individual ridges and valleys of a fingerprint are sensed. While the multitude of individual metal plates may perform the task of sensing contours of a finger pressed against a flat sheet, the large number of conductors and the interconnection arrangements needed to connect the sensor to electronic processing equipment detract from its utility in other sensing applications such as those in which the sensor must conform to nonplanar surfaces that are subjected to varying degrees of stress.
The article "A New Capacitive Transducer System for Measuring Force Distribution Statically and Dynamically" by Prof. K. Nicol appearing in the Proceedings of the Transducer Tempcon 81 Conference, dated June 10, 1981 discloses a matrix array of capacitors with elastic dielectrics used to determine force distribution by measuring capacitive change resulting from changes in the distances between the capacitance electrodes. While the Nicol arrangement substantially reduces the number of interconnections, each connection point to the electrode matrix must complete a conductive path. Consequently, making changes to the array requires a relatively complex procedure and reliability of the connection points is not assured. Additionally, a bridge type circuit is utilized to determine the capacitances of the matrix crossing which requires relatively complex analysis arrangements. It is an object of the invention to provide an improved tactile sensing arrangement adapted to conform to a variety of shapes and orientations, to be easily replaceable, and to perform wide range force imaging.